Heat-Exchanger using multi-stream approach
Cross-flow heat exchangers as shown in figure 1 are built for efficient heat transfer between two fluid streams without mixing them. In order to achieve this objective it is paramount to increase the surface area active for heat transfer. This usually results in designs which are comprised of channels which are much smaller than the distribution ducts. Hence meshing the whole domain of the equipment would yield very big meshes. However, in many situations the flow and heat transfer in individual channels can be described adequately using engineering correlations. In this case, the heat exchanger can be modelled as a pseudo three phase system composed of fluid 1, fluid 2 and solid where the volume fraction of each component is determined by the channel geometry and does not change in time. The motion of each fluid is then governed by the momentum equation for the fluid flow in an anisotropic porous medium. In this study, the problem has been solved by employing a topological mesh changes as depicted in figure 2. The heat exchanger is only present in the square middle section with four rectangular inlet/outlet sections attached to it. In state 1, fluid stream 1 which flows in horizontal direction is solved while blocking the sections of the mesh where only fluid 2 is present. In states 2 and 3, the topology is changed to accommodate fluid 2 and the solid. In each state, the relevant transport equations (turbulent Navier-Stokes and energy equation) are solved in segregated manner. The main advantages of this approach are optional block-solution of the three energy equations as well as efficient memory usage. Figures 3 to 8 show representative results for a simple three-dimensional geometry.
The effect of the non-uniform flow distributions on the resulting temperature distributions is clearly visible. These results can now be used to optimise the heat exchanger design.
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